ASAMBLEA COMUNTARIA PARA EL ADOQUINAMIENTO DE CALLE

The investigation in this work package is designed to monitor the surface ground motion (subsidence, uplift or stability) of the target area using line of slight (LOS) satellite radar interferometry (InSAR) prior to any permitted unconventional gas production in the Vale of Pickering. InSAR is an ideal technique for ground motion monitoring because

a) archive radar data (acquired by satellites since 1992) are available and can be utilised to ascertain a baseline of motion (or lack of motion) prior to any permitted gas operations, and

b) data from currently-orbiting satellites such as Sentinel-1A can be analysed to acquire information about the ongoing surface ground motion conditions in a region

c) the analysis produces data over a region rather than at a point location, which other techniques such as GNSS provide.

To date, the InSAR process has not been applied to monitoring energy operations in the UK because of the challenge of gaining coherence over non-urban areas. To resolve this challenge, we processed the data using the conventional SBAS (small baseline subset) process to gain precise results over urban areas and subsequently utilised the ISBAS (intermittent small baseline subset) process to acquire results over the non-urban areas. BGS has experience of applying InSAR to several ground surface monitoring applications in the UK e.g. utilising 55 ERS-1/2 images between 1992 and 1999 to investigate ground motion linked to ceased mining operations in south Wales (Bateson et al 2015). The InSAR technique was subsequently applied as part of the BGS-funded project to monitor environmental baseline conditions in Lancashire where planning applications were submitted by Cuadrilla in 2014 for the development of shale gas, see

www.bgs.ac.uk/research/groundwater/shaleGas/monitoring/lancashire.html. This has been

followed by its application across the Vale of Pickering.

The deliverable in this ground motion work package is to provide “an analysis of satellite (InSAR) data”. In order to achieve this, the following steps were followed:

• Obtain stacks of satellite SAR data

• Process the data using the SBAS InSAR techniques (thereby deriving results primarily for urban areas)

• Process the data to ISBAS level, thereby extending the results to non-urban areas) • Provide an analysis of the InSAR results.

Table 2. Analyses of InSAR processing for the Vale of Pickering.

Satellite Time period No. of scenes in the stack

Processing

mode Max velocity (mm/yr) Min velocity (mm/yr) ERS-1/2 1992-2000 72 SBAS +3.3 -3.1

ERS-1/2 1992-2000 72 ISBAS +6.2 -4.4

ENVISAT 2002-2009 25 SBAS +5.8 -4.4

The work package utilised the ISBAS technique of InSAR analysis as it has been found to provide results in non-urban areas where other InSAR techniques fail. The conventional SBAS technique requires that the target shows coherence in every image of the stack, while the ISBAS technique utilises coherence that is intermittent throughout the stack. Both SBAS and ISBAS processing and analysis was undertaken on each stack of radar images to provide results in urban and non-urban areas.

All of the available archive satellite radar data covering the time periods 1992-2000 and 2002-2009 over the Vale of Pickering were processed and analysed (Table 2).

2.3.2 Results of InSAR analysis in the Vale of Pickering.

Two sets of archive satellite radar data were acquired for the Vale of Pickering.

The archive radar data were acquired by the ERS-1/2 and ENVISAT satellites for the periods 1992-2000 and 2002-2009 respectively. These data were provided by the European Space Agency (ESA) to BGS under grant id.31573. Both stacks of data were analysed using SBAS and ISBAS InSAR techniques, i.e. four sets of analysis were undertaken and completed within this ground motion work package (Figure 20 and Figure 21).

There is no satellite coverage in the region between 2009 and 2014 due to the orbital decay of ENVISAT. No alternative commercial data are available to this study due to lack of acquisition in this time period. Nonetheless, we consider the period 1992–2009 is sufficient to provide a meaningful baseline assessment of ground motion prior to unconventional gas operations.

ARCHIVE RADAR SATELLITE DATA FROM 1992-2000

75 ERS-1/2 SAR scenes for 1992-2000 are available along satellite track 366 in descending mode. Of the 75 ERS-1/2 scenes in the archive, three were not used due to missing lines within the data. The results of the ERS-1/2 InSAR analysis are shown in Figure 20. Green areas are considered stable, red are subsiding on average over the time period, and blue are undergoing uplift.

The SBAS results are primarily constrained to urban areas (including roads) as these provided coherence in all of the radar images in the stack. It is apparent that the analysis shows that the area was predominantly stable between 1992 and 2000. There does appear to be a discrete zone of subsidence north of Whitby (in the Loftus area) but this is outside the Vale of Pickering monitoring area.

The ISBAS analysis of the ERS-1/2 radar data indicates that the majority of the area was stable. There are three zones of ‘dispersed’ uplift in this analysis, to the west, southwest and south of Scarborough. We believe that these zones in the ISBAS analysis are not related to geological motion (in our experience geological motions are more discrete), but are most likely due to vegetation changes and agricultural practices.

ARCHIVE RADAR SATELLITE DATA FROM 2002-2009

25 ENVISAT scenes for 2002-2009 along satellite track 366 in descending mode were ordered from ESA. 24 of the available 25 radar images were utilised; the scene acquired on 8th January 2005 was excluded due to failure to meet the baseline criteria. The relatively low number of scenes in the ENVISAT stack is a data limitation, and may have resulted in reduced compensation for some atmospheric effects. The results indicate a maximum velocity of 9.3 mm/year. The SBAS InSAR analysis comprises 72,697 points while the ISBAS analysis comprises 234,793 points (Figure 21).

Figure 20. InSAR SBAS (top) and ISBAS (bottom) analysis of ERS-1/2 satellite imagery (1992-2000) for the Vale of Pickering. Radar data supplied to BGS by ESA under grant id.31573. Contains Ordnance Data © Crown Copyright and database rights 2016. Licence number 100021290 EUL.

As with the 1992-2000 InSAR analysis, the SBAS results suggest that the urban areas are not affected by widespread subsidence or uplift, i.e. they are predominantly stable. There are some zones of dispersed uplift, notably along the coast southwards from Scarborough, and in the central and western extremes of the Vale of Pickering. The dispersed nature of the uplift suggests that they are not due to geological motion, and may not be genuine. They could be due to atmospheric effects.

The ISBAS analysis corroborates the SBAS analysis and provides additional results across the region. The ISBAS analysis identifies a discrete zone of subsidence to the south of the Vale of Pickering monitoring area and adjoining its southern boundary. This correlates with

InSAR SBAS results 1992-2000

Vale of Pickering Study Area

Vale of Pickering average ground

motion velocity (mm/yr)

-3.068000 - -2.500000 -2.499999 - -2.000000 -1.999999 - 2.000000 2.000001 - 2.500000 2.500001 - 3.307000

InSAR ISBAS results 1992-2000

Vale of Pickering Study Area

Vale of Pickering average ground

motion velocity (mm/yr)

-4.416000 - -2.500000 -2.499999 - -2.000000 -1.999999 - 2.000000 2.000001 - 2.500000 2.500001 - 6.259000

an area of compressible ground in the BGS GeoSure product, and we believe that this is genuine surface ground motion.

Within the Vale of Pickering ISBAS analysis there are significant zones of dispersed uplift. These do not seem to show any correlation with the zones of dispersed uplift in the ERS-1/2 ISBAS analysis, nor do they correlate with bedrock, superficial geology or compressible ground databases. Their dispersed nature suggests that they are not the result of geological motion. Due to the relatively small number of scenes in the stack they could be the result of atmospheric effects.

Further work will be undertaken up to end March 2016 to assess the analysis, particularly in relation to the zones of dispersed uplift and subsidence. The additional work will use the RMS point errors and the time series data for each point.

CURRENT SATELLITE RADAR DATA AND INSAR PROCESSING

It should also be noted that ESA launched a new radar satellite, Sentinel-1A, in April 2014. It was hoped that a sufficient stack of Sentinel-1A may have been available by the end of 2015 and it was proposed that BGS could have incorporate the data (when available) to establish current ground conditions in the study area. However to date the satellite has acquired less than 20 images in Interferometric Wide Swath Mode per stack, which is too low to guarantee high precision InSAR analyses. It is expected that a sufficient stack of imagery will be available by ~May 2016 and they could be processed to provide precise current ground motion information within a follow-on project.

PROBLEMS / ISSUES ENCOUNTERED AND EXPLANATION OF CAUSE(S)

There were no significant issues encountered. The ENVISAT and ERS-1/2 satellite radar data for the Vale of Pickering has been analysed using both SBAS and ISBAS InSAR techniques.

The number of scenes in the ENVISAT data stack for the Vale of Pickering is relatively low therefore the dispersed zones of uplift and subsidence may be due to some atmospheric effects that the processing could not filter out. This increased the uncertainty in the data (i.e. increased standard deviations). Further analyses of the dispersed zones will be completed by end March 2016 utilising a combination of i) the root mean square error within the data, ii) the time series and iii) expert elicitation.

A sufficient stack of Sentinel-1A data to undertake InSAR analyses will not be acquired by the satellite until ~May 2016.

2.3.3 Risks and mitigation measures

No additional risks have been identified.

2.3.4 Conclusions

The precursor monitoring project in the Lancashire area utilised SBAS and ISBAS InSAR analysis. The project identified that the majority of the area was stable while also identifying discrete areas of subsidence and uplift. It is proposed that the subsidence was due to both anthropogenic factors (coal mining) and natural factors (compressible peat sediments) while the zone of uplift was related to cessation of water extraction when mining ceased.

The Vale of Pickering ground motion analysis entailed processing two stacks of ERS-1/2 and ENVISAT radar satellite data using SBAS and ISBAS techniques (i.e. four levels of analysis in total). The ENVISAT data (2002-2009) consisted of 24 scenes. The SBAS analysis indicated that the urban areas were predominantly stable in the time period. The areas of dispersed motion in the SBAS and ISBAS analyses may be due to atmospheric

effects rather than genuine ground surface motion. Nevertheless, the zone of subsidence in the south of the monitoring area is thought to relate to compressible ground.

Figure 21. InSAR SBAS (top) and ISBAS (bottom) analysis of ENVISAT satellite imagery (2002-2009) for the Vale of Pickering. Contains Ordnance Data © Crown Copyright and database rights 2016. Licence number 100021290 EUL.

InSAR SBAS results 2002-2009

Vale of Pickering Study Area

Vale of Pickering average ground

motion velocity (mm/yr)

-4.443000 - -2.500000 -2.499999 - -2.000000 -1.999999 - 2.000000 2.000001 - 2.500000 2.500001 - 5.816000

InSAR ISBAS results 2002-2009

Vale of Pickering Study Area

Vale of Pickering average ground

motion velocity (mm/yr)

-7.346000 - -2.500000 -2.499999 - -2.000000 -1.999999 - 2.000000 2.000001 - 2.500000 2.500001 - 9.324000

The ERS-1/2 Vale of Pickering dataset comprised 72 satellite radar scenes and it has therefore not been affected by atmospheric conditions. The SBAS analysis revealed that the urban areas and connecting roads are stable i.e. they are not affected by regional subsidence or uplift between 1992 and 2000. The ISBAS analysis also indicated that the area is predominantly stable apart from three zones that appear to display dispersed uplift. Our experience of this type of dispersed result is that it is not due to geological motion (which is more discrete) but it is most likely due to vegetation changes and agricultural practices. Finally, ESA launched the Sentinel-1A in April 2014 but it did not acquire a sufficient stack of images to allow a precise InSAR analysis within the timescale of this monitoring project. It is expected that a stack will be available by ~May 2016, which will allow the current surface ground motion conditions to be monitored precisely.

2.3.5 References

Bateson, Luke; Cigna, Francesca; Boon, David; Sowter, Andrew. 2015 The application of the Intermittent SBAS (ISBAS) InSAR method to the South Wales Coalfield, UK. International Journal of Applied Earth Observation and Geoinformation, 34. 249-257.

2.4 ATMOSPHERIC COMPOSITION